A known method for producing expandable polystyrene polymers, hereinafter referred to as EPS, is by aqueous suspension polymerization. It is typically a batch process where two or more monomer-soluble polymerization initiators are used with a rising stepwise, continuous, or combination temperature profile. Initiators for the process are selected on the basis of their half life temperatures to provide a measured supply of radicals at selected points along the temperature profile such that effective conversion occurs within an acceptable period of time. For styrene polymerization, it is convenient to describe initiator decomposition performance in terms of one hour half life temperature, defined as that temperature sufficient to cause decomposition of one half of the starting concentration of initiator over a one hour time period.
Traditionally, suspension polymerization to prepare EPS is conducted in a process using two different temperature stages and two initiators, a first stage initiator and a second stage or “finishing” initiator, with different half life temperatures, each appropriate for the particular temperature stage. In such a process, dibenzoyl peroxide (BPO) is often used as the first stage initiator at a reaction temperature of about 82° C. to 95° C. Other first stage initiators useful in this temperature range might include tertiary butyl peroxy-2-ethylhexanoate, tertiary amyl peroxy-2-ethylhexanoate and 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane. Initiators such as tertiary butyl peroxybenzoate (TBP) or dicumyl peroxide (DCP) are widely used for the higher temperature stage, or second stage, at 115° C. to 135° C. The second stage is usually a finishing step intended to minimize residual monomer in the EPS. In commercial processing, this stage is often held above 125° C. for prolonged intervals to reduce monomer content to acceptable levels.
Characteristic shortcomings of the traditional process are long reaction times necessary to obtain adequate conversion in the first stage and relatively high finishing temperatures required in the second stage.
In view of alleviating this drawback, document U.S. Pat. No. 6,608,150 has proposed an improved process for the preparation of expandable polystyrene in which “intermediate” temperature peroxides, that is to say initiator peroxides having a one hour half life temperature ranging from 101° C. to 111° C., are used. According to this document, the use of such “intermediate” peroxides in the preparation of expandable polystyrene allows to reduce the conversion time for some hours.
Expandable polystyrene, as prepared in the suspension process, is in the form of essentially spherical beads. It is mentioned in U.S. Pat. No. 6,608,150 that some blowing agent, such as pentane, may be used in the process in order to render the polystyrene beads expandable.
However, it is known that the use of pentane in a process of preparation of expandable polystyrene leads to a decrease in the molecular weight as a result of transfer mechanism on the alkane agent (see Principles of Polymerization-George Odian p 203-217, McGraw-Hill Book Company 1970).
Additionally, it is more and more requested in the industry that the polystyrene resins comprise flame retardant additives for safety reasons. For instance, flame retarded resins are widely used in insulation applications. The addition of such additives may also lead to a decrease of the molecular weight of the resulting polystyrene.
It appears then that several factors may lead, in the preparation process of expandable polystyrene, to a significant decrease of the molecular weight of such expandable polystyrene.